In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a single or few 2-dimensional layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
In collinear holographic data storage the object beam and one or more reference beams run along a common optical axis. For this purpose an optical system for combining the object beam and the one or more reference beams on the common optical axis is needed. In the following this optical system will be called a “reference-beam coupler”.
For example, WO 2006/003077 discloses a collinear optical head for a reflection-type holographic recording medium, which is capable of obtaining an increased capacity. The optical head includes a special reference beam coupler, which consists of a 6f lens system, a special twin diffractive beam generator and a specially shaped Fourier filter. The reference beam coupler will be described in more detail later on with reference to FIGS. 1 and 2.